Study On The Key Technology Of OFDM Based On Emergency Communication Wireless LAN

Emergency communication system supply temporary and mobile emergency communication in condition of combat readiness, disaster relief and communication network failure. To grasp the condition of disaster spot, modern emergency communication system needs higher demands for image and data transmission services voice, data and image to users under emergency and disaster environment. Wireless Local Area Network (WLAN) whose user access network through wireless channel can support wideband transmission for image and data. Access Internet by mobile vehicle transceiver who employs wireless local area network (WLAN) is widely used in emergency communication system. Due to its high transmission data rate and anti-multipath interference capability, orthogonal Frequency Division Multiplexing (OFDM) is selected as key technology for emergency WLAN which good adapts to complex channel condition and meet an uncertain bandwidth demand.Orthogonal sub carriers in OFDM system yield a lot of technical issues in the practical process of OFDM. This thesis bases on emergency WLAN and focuses on OFDM system. The research work involves several key technology of OFDM WLAN, including synchronization algorithm, channel estimation and PAPR reduction technology. A revised delay and auto-correlation timing synchronization algorithm is proposed while channel estimation based on subspace theory and its FPGA realization method is given. A new PAPR reduction algorithm based on ACE-POCS is posed.As a basic technology, OFDM system model is analysis first. The characteristic of OFDM system is given. Simulation results are proposed to consider the influence system parameters on system performance.System synchronization algorithm for emergency WLAN system whose channel condition is complicated needs a large CFO estimation range while the estimation accuracy is ensured. Existed system synchronization methods are given in this paper. Half periodic algorithm is proposed to realize roughly estimation process. Compare to auto-correlation timing synchronization algorithm, proposed algorithm increase the estimation range to±1.25MHz and decrease the calculation amount. A two stages strategy which combined with the improved coarse frequency offset estimation algorithm and fine estimation algorithm is used to realize system synchronization. The contradiction between estimation range and estimation accuracy is solved. Simulation results show that proposed algorithm adapts to various channel conditions of emergency WLAN and its practical value is high.Channel estimation for emergency WLAN needs solve the conflict between the performance and complexity due to its high demand for spectral efficiency. A new OFDM channel estimation algorithm based on signal/noise subspace theory is proposed and a new low-rank order choosing criterion is raised. Simulation results show that proposed algorithm meet the environmental adaptability demand for emergency WLAN. The realization method of channel estimation based on single value decomposition (SVD) is analyzed. And parallel Jacobi algorithm is used to fulfill matrix diagonal progress. Hardware design of CORDIC processor is given. The FPGA realization scheme occupies a little system resource and is good for practical progress.High peak-to average power ratio (PAPR) reduction method selected by emergency WLAN should cause no BER loss. Existed PAPR reduction methods are analysis first. Mathematic model for Active Constellation Extension (ACE) which causes no BER loss is presented. Based on ACE method, a revised algorithm based on projection onto convex sets (POCS) is proposed. Simulation results show that proposed algorithm can speed up convergence by a new method for constellation reconstruction. Weight Jaccobian matrix is used to fulfill power back off procedure. Based on revised ACE-POCS and weight Jaccobian matrix, a two step procedure is pot forward to reduce system PAPR. Simulation results show that proposed algorithm can definitely reduce PAPR value to the given threshold and meet the demand of low PAPR transmission for emergency WLAN.